Electrochemical apparatus having tin-based cathodic catalyst

ABSTRACT

An electrochemical electrode comprising a tin-based catalyst, method of making, and method of use are provided. Catalyst particles are prepared which comprise tin deposits of about 0.1 nm to about 10 nm deposited onto carbon support. Preparing an ink comprising the catalyst particles and a binder enable an electrode to be prepared comprising the catalyst particles bound to an electrode substrate. The electrode may then be used in an apparatus and process to reduce carbon dioxide to products such as formate and formic acid at Faradaic Efficiencies up to 95 percent.

FIELD

This description relates generally to electrodes for electrochemicalreactions, particularly to cathodic electrodes comprising tin-basedcatalyst, and more particularly to such electrodes for theelectrochemical reduction of carbon dioxide to formate and formic acid.Also included are methods of making and using such electrodes.

BACKGROUND

Electrochemical reactions involve the use of an electric current toeffect chemical reactions. In a typical arrangement, an anode immersedin an anolyte and a cathode immersed in a catholyte are separated by amembrane, generally an ion exchange membrane. An oxidation reactiontakes place at the anode and a reduction reaction takes place at thecathode. For example, with the electrochemical reduction of carbondioxide, carbon dioxide is reduced to formate and formic acid at thecathode and oxygen is evolved from water at the anode. In someapplications, carbon dioxide is introduced into a catholyte compartmentthrough a porous cathode which porous cathode comprises a cathodiccatalyst adapted to reduce carbon dioxide to formate and formic acid.

Cathodic and anodic catalysts play important roles in determining whatreactions occur at the cathode and at the anode, respectively, as wellas the electrolytic efficiency of such reactions. Efficiency istypically measured as Faradaic Efficiency (FE) and is the efficiencywith which electrons (charge) are transferred. That is, the percent ofthe total current that passes through the electrochemical cell that isused to produce the desired product (e.g., formate). In addition, it isdesirable to achieve high current density in stable, long-term use.Tin-based cathodic catalysts, which generally exist as tin, tin oxide(II) (SnO), and tin oxide (IV) (SnO₂) (collectively, SnO_(X)), have beenshown to give good results in the selective electrochemical reduction ofcarbon dioxide to formate and formic acid. In addition to selectivity toformate and formic acid, desirable characteristics of such tin-basedcathodic catalysts include high surface area of the catalyst on thecathode, physical and mechanical stability, high FE, and high and stablecurrent density during operation. Finally, while catalyst activity canbe unpredictable and is highly dependent upon structure, morphology, andelectrolysis conditions, other post-transition metals such as lead andindium may also be candidates.

BRIEF SUMMARY OF THE INVENTION

Electrochemical electrodes comprising tin-based catalysts, method ofmaking, and method of use are disclosed. Particularly, the electrodescomprise tin-based nano-scale deposits on carbon support to produce Sn—Cparticles and such Sn—C particles are applied to a support substrateusing a binder. In use, the resulting electrodes provide stable highcurrent density and high FE or selectivity over time.

The catalysts are prepared by depositing tin onto a carbon support suchas carbon black [e.g., Vulcan® XC72 (Cabot Corp., Boston, Mass.)] usinga solution of tin (II) chloride (SnCl₂), ethylene glycol, carbon black,and water. After recovery, the resultant Sn—C catalyst particles arecombined with a binder such as a sulfonated tetrafluoroethylene-basedfluoropolymer-copolymer (e.g., Nafion®, The Chemours Co., Wilmington,Del.) or compounds comprising tetrafluoroethylene groups [e.g.,polytetrafluoroethylene (PTFE)] and applied to a support substrate suchas carbon fiber paper (e.g., TORAY® Carbon Paper 120, Toray Industries,Inc., Tokyo Japan). The finished electrodes are used in an electrolyticprocess to reduce carbon dioxide to formate and formic acid.

In one exemplary embodiment, an apparatus is provided for theelectrochemical reduction of carbon dioxide to formate. The apparatusincludes an anolyte compartment, the anolyte compartment at leastpartially defined by an anode and a membrane, a catholyte compartment,the catholyte compartment at least partially defined by the membrane anda cathode, and a gas compartment, the gas compartment in fluidcommunication with the cathode. The cathode comprises a porous substrateand a catalytic coating at least partially covering the substrate. Thecatalytic coating is applied to the substrate using a process comprisingthe steps of (a) mixing catalyst powder, the catalyst powder comprisingtin-carbon particles, the tin-carbon particles comprising 0.1 nm to 10nm tin deposits on carbon support, an alcohol-based solvent, andpolymeric binder, to form a catalyst ink, (b) applying the ink to thesubstrate, and (c) drying the ink to the substrate.

In a further exemplary embodiment, the tin deposits comprise greaterthan 22 weight percent of the catalyst powder.

In a further exemplary embodiment, the substrate is carbon fiber paper,the catalyst loading is between 0.1 mg/cm² and 10 mg/cm², and the bindercontent is between 0.1 to 5 weight percent of the catalyst loading.

In a further exemplary embodiment, utilizing the apparatus for theelectrochemical reduction of carbon dioxide to formate, a processcomprises the steps of feeding an appropriate anolyte into the anolytecompartment, (b) feeding an appropriate catholyte into the catholytecompartment, (c) feeding carbon dioxide into the gas compartment,through the cathode, and into the cathode compartment, and (d)impressing a voltage between the anode and the cathode. In a furtherexemplary embodiment, the impressed voltage is sufficient to provide aV_(CATHODE) range between −1.5 and −2.0 V_(SCE). In a further exemplaryembodiment, the Faradaic Efficiency of carbon dioxide to formate isabout 55 to 95 percent over a period of over 100 hours.

In another exemplary embodiment, a process is provided comprising thesteps of (a) preparing catalyst powder by (i) mixing, proportionally,0.84 gm tin chloride (II), 200 ml ethylene glycol, 2 ml deionized water,and 0.20 gm carbon support, (ii) refluxing the mixture of Step (a)(i) atbetween 170 deg. C. and 200 deg. C. for about three hours, (iii) coolingthe refluxed material, and (iv) filtering and drying the cooled,refluxed material to obtain the catalyst powder, (b) preparing acatalyst ink by mixing, proportionally, 0.02 gm catalyst powder, 100 mlisopropyl alcohol, and between 0.02 mg and 1 mg sulfonatedtetrafluoroethylene-based fluoropolymer-copolymer binder, (c) spraying aportion of catalyst ink uniformly onto a porous substrate, (d) allowingthe catalyst ink sprayed onto the porous substrate to dry, and (e)repeating Steps (c) and (d) until a desired coating weight is achieved.

In a further exemplary embodiment, an apparatus is provided comprisingan anolyte compartment, which anolyte compartment is at least partiallydefined by an anode and a membrane, a catholyte compartment, whichcatholyte compartment is at least partially defined by the membrane anda cathode, and a gas compartment, the gas compartment in fluidcommunication with the cathode. The cathode is prepared by a processcomprising the steps of (a) preparing catalyst powder by (i) mixing,proportionally, 0.84 gm tin chloride (II), 200 ml ethylene glycol, 2 mldeionized water, and 0.20 gm carbon support, (ii) refluxing the mixtureof Step (a)(i) at between 170 deg. C. and 200 deg. C. for about threehours, (iii) cooling the refluxed material, and (iv) filtering anddrying the cooled, refluxed material to obtain the catalyst powder, (b)preparing a catalyst ink by mixing, proportionally, 0.02 gm catalystpowder, 100 ml isopropyl alcohol, and between 0.02 mg and 1 mgsulfonated tetrafluoroethylene-based fluoropolymer-copolymer binder, (c)spraying a portion of catalyst ink uniformly onto a porous substrate,(d) allowing the catalyst ink sprayed onto the porous substrate to dry,and (e) repeating Steps (c) and (d) until a desired coating weight isachieved.

In another exemplary embodiment, a process is provided comprising thesteps of (a) preparing a catalyst powder by (i) mixing, proportionally,tin chloride (II) and carbon support in a ratio of about 4.5 gm/gm andsufficient ethylene glycol and water to maintain a dilute slurry, (ii)refluxing the mixture of Step (a)(i) at between 170 deg. C. and 200 deg.C. for about three hours, (iii) cooling the refluxed material, and (iv)filtering and drying the cooled, refluxed material to obtain thecatalyst powder (b) preparing a catalyst ink by mixing, proportionally,catalyst powder and isopropyl alcohol in a ratio of about 0.02 gm/100 mlwith a suitable binder, which binder is proportional to the isopropylalcohol in a ratio of about 0.02 mg to 1 mg per 100 ml isopropylalcohol, for about 30 minutes, (c) spraying a portion of the catalystink uniformly onto a porous substrate, (d) allowing the catalyst inksprayed onto the porous substrate to dry; and (e) repeating Steps (c)and (d) until a desired coating weight is achieved.

In another exemplary embodiment, an apparatus is provided whichapparatus includes an anolyte compartment, the anolyte compartment atleast partially defined by an anode and a membrane, a catholytecompartment, the catholyte compartment at least partially defined by themembrane and a cathode, and a gas compartment, the gas compartment influid communication with the cathode. The cathode is prepared by aprocess comprising the steps of (a) preparing a catalyst powder by (i)mixing, proportionally, tin chloride (II) and carbon support in a ratioof about 4.5 gm/gm and sufficient ethylene glycol and water to maintaina dilute slurry, (ii) refluxing the mixture of Step (a)(i) at between170 deg. C. and 200 deg. C. for about three hours, (iii) cooling therefluxed material, and (iv) filtering and drying the cooled, refluxedmaterial to obtain the catalyst powder (b) preparing a catalyst ink bymixing, proportionally, catalyst powder and isopropyl alcohol in a ratioof about 0.02 gm/100 ml with a suitable binder, which binder isproportional to the isopropyl alcohol in a ratio of about 0.02 mg to 1mg per 100 ml isopropyl alcohol, for about 30 minutes, (c) spraying aportion of the catalyst ink uniformly onto a porous substrate, (d)allowing the catalyst ink sprayed onto the porous substrate to dry; and(e) repeating Steps (c) and (d) until a desired coating weight isachieved.

In a further exemplary embodiment, utilizing the apparatus describedjust above, a process comprises (a) feeding an appropriate anolyte intothe anolyte compartment, (b) feeding an appropriate catholyte into thecatholyte compartment, (c) feeding carbon dioxide into the gascompartment, through the cathode, and into the cathode compartment, and(d) impressing a voltage between the anode and the cathode. In a furtherexemplary embodiment, the impressed voltage is sufficient to provide aV_(CATHODE) range between −1.5 and −2.0 V_(SCE). In a further exemplarembodiment, the Faradic Efficiency of carbon dioxide to formate is about55 to 95 over a period of over 100 hours.

BRIEF DESCRIPTION OF THE SEVERAL FIGURES

The invention will be more readily understood by reference to theaccompanying figures. The figures are incorporated in, and constitute apart of, this specification, illustrate several embodiments consistentwith the invention and, together with the description, serve to explainthe principles of the invention. For purposes of illustration, drawingsmay not be to scale.

FIG. 1 is a graphical representation of Current Density i (mA/cm²) v.Cell Potential (−V) for Sn—C-1 particles.

FIG. 2 is a graphical representation of FE v. Cell Potential (−V) forSn—C-1 particles.

FIG. 3 is a schematic diagram of an experimental test apparatus.

FIGS. 4-9 are transmission electron microscopy (TEM) images of Sn—C-2particles from Sn—C Experiment 2.

FIGS. 10 and 11 are TEM images of Sn—C-3 particles from Sn—C Experiment3.

FIG. 12 is a graphical representation of Current Density i (mA/cm²) v.V_(ANODE) (V_(SCE)) for two runs each of Test Protocol 1 and TestProtocol 2, each protocol using Sn—C-3.

FIG. 13 is a graphical representation of Current Density (mA/cm²) v. IR(V or V_(SCE)) for two runs each of Test Protocol 1 and Test Protocol 2.

FIG. 14 is a graphical representation of I_(TOTAL) (mA/cm²), FE (%), andV_(CATHODE) v. Time (t) (hrs.) for Electrode Experiment S1.

FIGS. 15-18 are TEM images of unused and used Sn—C-3 particles fromElectrode Experiment S1.

FIG. 19 is a graphical representation of i_(TOTAL) (mA/cm²), FE (%), andV_(CATHODE) V. Time (t) (hrs.) for Electrode Experiment S2.

FIGS. 20-23 are TEM images of unused and used Sn—C-3 particles fromElectrode Experiment S2.

FIG. 24 is a graphical representation of Current Density (mA/cm²) andFormate Product Selectivity (%) v. Time (t) (days) for ElectrodeExperiment S3.

FIGS. 25-28 are TEM images of unused and used Sn—C-3 particles fromElectrode Experiment S3.

In describing the various embodiments of the invention, specificterminology will be resorted to for the sake of clarity. However, it isnot intended that the invention be limited to the specific terms soselected and it is to be understood that each specific term includes alltechnical equivalents which operate in a similar manner to accomplish asimilar purpose.

DETAILED DESCRIPTION

Preparation of Sn—C Catalyst Particles

Catalyst particles of nano-sized tin deposited on carbon powders (Sn—C)were prepared using the procedure shown in Table 1 below.

TABLE 1 (1) mix predetermined amounts of deionized water and ethyleneglycol; (2) add predetermined amounts of tin chloride (II) and ethyleneglycol and ultrasonicate (e.g., ten minutes); (3) add a predeterminedamount of carbon black and ultrasonicate (e.g., 30 minutes); (4) refluxthe mixture at a predetermined temperature (e.g., 196 deg. C.) for apredetermined time (e.g., three hours) while stirring; (5) allow themixture to cool to room temperature; (6) vacuum filter the solutionthrough a 0.2 micron (μm) membrane; (7) immerse the membrane with thecake into 50 ml isopropyl alcohol (IPA) and ultrasonicate (e.g., tenminutes) until all black powder is removed; (8) heat black powder/IPAmixture to evaporate the IPA; (9) scrape out resulting black powder; 10)calculate final weight percent SnO_(X) {[(final weight Sn—C catalystpowder) − (initial weight carbon black powder)]/final weight Sn—Ccatalyst powder} * 100

The carbon black used was Vulcan® XC72 (Cabot Corp., Boston, Mass.)having a nominal particle size of 1 micron. Typical surface area isabout 250 m²/gram. Other suitable carbon-based support materials includecarbon nanowires and carbon nanotubes.

Sn—C Experiment 1 (Sn—C-1)

Using the procedure shown in Table 1, Sn—C particles were prepared usingthe ingredients and recipe shown in Table 2 below.

TABLE 2 Compound Amount Tin Chloride (II) 5.00 gm Ethylene Glycol 400 mlDeionized Water 15 ml Carbon Black 2.00 gm

The mixture was refluxed at 170 deg. C. for four hours, cooled, andfiltered to obtain the Sn—C particles (Sn—C-1). Electrodes were preparedusing a modification of the electrode preparation protocol outlinedherein below in Table 6. The significant difference in the modifiedelectrode preparation protocol was that no binder was used to adhere theSn—C-1 particles to the electrode substrate [i.e., carbon fiber paper(CFP)]. This allowed measurement of the performance of the catalystparticles themselves in terms of FE (%) as a function of applied cathodevoltage without the possible complex effects due to the presence of abinder. While not wishing to be bound by any particular theory, it isbelieved that without a binder to adhere the Sn—C-1 particles to the CFPsubstrate, the current density [i (mA/cm²)] may decrease continuouslywith Sn—C-1 particles being washed away in the cell. For whatevercurrent is noted, however, the FE (%) can be calculated. Thus, the FE(%) so obtained may be compared with that observed where solid Sn ortin-electroplated CFP electrodes are tested (i.e., 70-90% FE).

As shown in FIG. 1, at total cell voltages between 3.35 and 3.95 (−V),the resulting catalyst exhibited low current densities. While notwishing to be bound by any particular theory, it is believed that in theabsence of a binder to hold the particles to the CFP substrate, theparticles did not adhere well. However, as shown in FIG. 2, at thechosen cell voltage range, FE is well-maintained.

Sn—C Experiment 2 (Sn—C-2)

Using the procedure shown in Table 1, Sn—C particles were prepared usingthe ingredients and recipe shown in Table 3 below.

TABLE 3 Compound Amount Tin Chloride (II) 0.84 gm Ethylene Glycol 200 mlDeionized Water 2 ml¹ Carbon Black 0.60 gm ¹1% of ethylene glycol volume

The mixture was refluxed at 196 deg. C. for three hours, cooled, andfiltered to obtain the Sn—C-2 particles (Sn—C-2). Transmission electronmicroscopy (TEM) images for the resulting Sn—C-2 particles are shown inFIGS. 4-9. Tin-based crystallites of about 5 nm are distributed on thecarbon support. The percent tin-based material, comprising SnO_(X), wasabout 22 weight percent.

Sn—C Experiment 3 (Sn—C-3)

Using the procedure shown in Table 1, Sn—C particles were prepared usingthe ingredients recipe shown in Table 4 below.

TABLE 4 Compound Amount Tin Chloride (II) 0.84 gm Ethylene Glycol 200 mlDeionized Water 2 ml Carbon Black 0.20 gm

The mixture was refluxed at 196 deg. C. for three hours, cooled, andfiltered to obtain Sn—C particles (Sn—C-3). Using less carbon, byproportion, than Sn—C Experiment 2 (Sn—C-2), the percent tin-basedmaterial was about 30 weight percent. TEM images for the resultingSn—C-3 particles are shown in FIGS. 10-11. Tin-based crystallites ofabout 5 nm are distributed on the carbon support.

Sn—C Experiment 4 (Sn—C-4)

Using the procedure shown in Table 1 and the recipe shown in Table 4, aneffort was made to increase the size of the tin-based nano deposits onthe carbon support to the range of 10 nm. Such size variation isdependent upon such factors as the time of hydrothermal processing, thewater:ethylene glycol ratio, and the concentration of tin chloride. Theprocedure was the same as that for Experiment 3, except that the time ofhydrothermal processing was six hours, instead of three. The results,shown below in Table 5, were disappointing, however, as the resultingcatalytic electrodes exhibited very low FE (i.e., 33 to 60 percent).

TABLE 5 Formate Time Cell Potential Current Density Current Density FE(hrs) (−V) [i (mA/cm²)] [i (mA/cm²)] (%) 0 3.75 106.0 33.4 31.6 1 3.75100.5 59.5 59.2 3 3.75 98.5 51.6 52.4 24.5 3.75 92.0 40.2 43.7Electrode Preparation

Electrodes were prepared based upon Sn—C-3 particles prepared accordingto Sn—C Experiment 3 above. A porous and electrically conductiveelectrode substrate was used which comprised carbon fiber paper (CFP).The CFP used in these experiments was TORAY® Carbon Paper 120 which hada thickness of about 350 microns and a porosity of about 80 percent. TheCFP microstructure consists of carbon fibers about 7-10 microns indiameter and are held together with polytetrafluoroethylene (PTFE)binder. A catalyst ink comprising Sn—C-3 particles was prepared using asulfonated tetrafluoroethylene-based fluoropolymer-copolymer (e.g.,Nafion®). As supplied, the Nafion® binder was a 5 percent w/w solutionin aliphatic alcohols, principally isopropyl, and water, from SigmaAldrich. This solution was further diluted with water to form a solutionwith a binder concentration of about 2 mg/ml. Requisite amounts of thisdiluted solution was used to produce electrodes with a binderconcentration between 0.03 percent w/w and 5 percent w/w of catalystloading. The electrodes were prepared according to the procedure shownin Table 6 below.

TABLE 6 (1) prepare a catalyst ink mix by ultrasonicating predeterminedamounts of catalyst powder (e.g., 0.02 gm) prepared as above, isopropylalcohol (e.g., 100 ml), and a requisite amount of a solution containingsulfonated tetrafluoroethylene-based fluoropolymer-copolymer binder(e.g., Nafion ®) to provide the desired binder loading for a period oftime (e.g., 30 minutes); (2) prepare an electrode substrate from asection of carbon fiber paper (CFP); (3) spray catalyst ink uniformlyonto the CFP until just wet and immediately place sprayed electrode ontohot plate until dry (e.g., few seconds)¹; (4) repeat (3) until desiredcoating weight is achieved; (5) calculate coating weight per unit area:(coating weight)/(geometric area of electrode) ¹Attempts to apply largeamounts of ink in one coating may result in Sn—C particle “clumps” whichinhibit catalytic effectiveness.

Best results are obtained if thick coatings as well as low catalystsloadings are avoided. Thicker coatings may crack and fall away from thesubstrate surface, particularly due to physical abrasion caused byflow-through carbon dioxide. Low loading, on the other hand, may not besufficient to cover the entire substrate surface which a resulting lossof efficiency. Experimental catalyst loadings ranged from 0.45-4.5mg/cm² with varying binder concentrations of 0.03-5 weight percent.Catalyst loadings are calculated as shown in Table 6. While not wishingto be bound by any particular theory, it is believed that excess binderproduces an undesirable overload of the binder onto the Sn—C particleswhich effectively blocks the surface area of the Sn—C particlesavailable for catalyst activity. Too little binder may reduce theability of the Sn—C particles to adhere to the CFP substrate surface.

To study the operational effects of the various electrodes as cathodes,it was necessary to obtain measurements of the cathode voltage. Sincethe cathode voltage cannot be directly applied, continuous, single-passexperiments were conducted where applied cell voltage was monitored andcontrolled such that the resulting cathode voltage was kept as close apossible to a predetermined value using the following formula:V _(APPLIED-TOTAL-CELL) =V _(CATHODE) +IR+V _(ANODE),   (1)where V_(ANODE)=anode voltage at the prevailing current density, i(mA/cm²) and IR=cell internal resistance or ohmic potential drop in thesolution at the prevailing current density, i (mA/cm²). Experiments wereperformed to determine IR and V_(ANODE) so that V_(CATHODE) could becalculated from Eq. (1).

A general schematic of the apparatus used is shown in FIG. 3. Athree-compartment electrochemical reactor 350 is shown in which athree-compartment container 12 encloses an anolyte compartment 18, ananode 14, and, during operation, anolyte 24 contained within the anolytecompartment 18; a membrane 22; a catholyte compartment 20, a porouscathode 16 as described herein, and, during operation, catholytecompartment mixture 26 contained within the catholyte compartment 20;and a gas compartment 28, the gas compartment 28 containing, duringoperation, CO₂ gas 30. The membrane 22 separates the anolyte compartment18 and the catholyte compartment 20 and the porous cathode 16 separatesthe catholyte compartment from the gas compartment 28. Also duringoperation, an anolyte feed 36 introduces anolyte 24 into the anolytecompartment 18, an anolyte withdrawal 44 removes anolyte 24 as well asother anode reaction products, a catholyte feed 34 introduces catholyte25 into the catholyte compartment 20, a catholyte compartment mixturewithdrawal 42 removes catholyte compartment mixture 26, and a CO₂ gasfeed 32 introduces CO₂ gas 30 into the gas compartment 28. Duringoperation, the CO₂ gas 30 in the gas compartment 28, under a pressuredifferential across the porous cathode 16, is distributed (indicated byarrows 40 and flows through the porous cathode 16 and into the catholytemixture 26.

To obtain IR and V_(ANODE), two experiments were performed withreference electrode probes extended into the cathode and anode chambers.The potential difference between the probes on either side of a membrane(e.g., Nafion®) was measured and the potential difference between thecathode and the probe in the respective chamber was also measured. AnAg—AgCl reference electrode was inserted from the side into the cathodechamber and a Cu—CuSO₄ reference electrode was inserted into the anodechamber.

With reference to FIG. 3, Test Protocol 1 was performed using aSn-electroplated CFP cathode, 2M KCl catholyte, and 0.5M H₂SO₄+0.5MK₂SO₄ anolyte. Test Protocol 2 was performed using a nano-Snparticle-based cathode (1.8 mg/cm²); the catholyte and anolyte wereidentical to Test Protocol 1. The anode comprised a Ti substrate coatedwith a IrO₂. Two runs were made of each set of test conditions.

For the two test protocols of two runs each, FIG. 12 shows CurrentDensity i (mA/cm²) V. V_(ANODE) (V_(SCE)). FIG. 13 shows Current Densityi (mA/cm²) v. IR (V or V_(SCE)) [ohmic loss (V or V_(SCE))]. After thetests, since V_(APPLIED-TOTAL-CELL) was measured directly, V_(CATHODE)was obtained using Eq. (1). Although shown in FIGS. 12 and 13, TestProtocol 2 results were not used in the calculations as there was alarge loss in current densities. While not wishing to be bound by anparticular theory, this large loss in current densities may have beencaused by degradation of the particle-coated cathode, perhaps due, inpart, to mechanical abrasion from the reference electrode.

Post-experiment reference probe measurements versus SCE in saturated KClare shown in Table 7 below, which indicates very little degradation overtime.

TABLE 7 Probe Type Before (mV) After (mV) Ag—AgCl −42 −28 Cu—CuSO₄ 72 50

Table 8 shows the results of a series of experiments performed withSn—C-3 cathode electrodes made according to Experiment 3 above. Catalystloading was 1.8 mg/cm² at 30 weight percent SnO_(X) on CFP substrate. Inall experiments, the catholyte was 2M KCl at 9-11 ml/min. saturated withcarbon dioxide. The anolyte was H₂SO₄+K₂SO₄ at 55-65 ml/min. The carbondioxide flowrate was about 100 ml/min. The anode comprised a mixed metaloxide anodic catalyst (IrO₂) on a Ti substrate.

TABLE 8 Sn—C-3 Experiment Parameter Units S1 S2 S3 Run Time hours 120200 288 V_(CATHODE) (min-max) V_(SCE) −1.77 to −1.89 −1.63 to −1.72−1.66 Range V_(CATHODE) (expected) V_(SCE) 1.85 1.7 1.66 i_(TOTAL) (t =0) mA/cm² 200 140 127 i_(TOTAL) (t = max) mA/cm² 170 101 114 Decrease in{[i_(TOTAL) (t = 0) − i_(TOTAL) 15 40 10 Current (%) (t =max)]/i_(TOTAL) (t = 0)} * 100 FE (%) (t = 0) 80-87 67-77 70 FE (%) 6863 70 (t = max) Catalyst Loading mg/cm² 1.8 1.8 1.8 Binder Contentweight percent of catalyst 0.1 0.5 1 loading

Sn—C-3 Electrode Experiment S1

As shown in Table 7, V_(CATHODE) is in the range of −1.77 to −1.89V_(SCE) with the majority of the experiment at V_(CATHODE)>−1.8 V_(SCE).FIG. 14 shows i_(TOTAL) (mA/cm²), FE (%), and V_(CATHODE) V. Time(hrs.). One could conclude, from FIG. 14 that, with time, the cathodevoltage (V_(CATHODE)) must be increased to maintain the same currentdensity (i_(TOTAL)). Notably, at about t=115 hours, V_(CATHODE) is aboutthe same as V_(CATHODE) at t=20, but there is about a 15 percent loss ini_(TOTAL) and an 18.5 percent loss in FE. A more abrupt loss in currentdensity (i_(TOTAL)) occurs at about t=120 hours. A comparison ofassociated TEM images shown in FIGS. 15-18 shows a possible loss ofSn—C-3 particles with time from the CFP support surface. While notwishing to be bound by any particular theory, physical abrasion couldcause this loss, which may be reduced by adjusting binder content. Onthe other hand, such loss could be due to the higher appliedV_(CATHODE). (Compare, Sn—C-3 Electrode Experiments S2 and S3, usinglower V_(CATHODE).) FIG. 14 also indicates a sharp rise in V_(CATHODE)as well as i_(TOTAL) from t=0 up to about t=20 hours, possible due, inpart, to some preconditioning effects.

Sn—C-3 Electrode Experiment S2

Turning now to FIG. 19, V_(CATHODE) is in the range of −1.65 to −1.72V_(SCE) with the majority of the experiment at V_(CATHODE)≤−1.70V_(SCE). As seen in FIG. 8, a large drop in total current density,i_(TOTAL), of about 25-40 percent is observed between t=25 hours andt=100 hours and then remains constant from about t=100 hours to aboutt=200 hours. While not wishing to be bound by any particular theory, thefluctuations in V_(CATHODE) and FE in the last 100 hours may have beencaused by poor adhesion of the particles to the CFP substrate.

Turning now to related FIGS. 20-23, the TEM images show no loss in Sn—Cparticle density for used versus unused. Thus, applying V_(CATHODE) ofabout −1.7 V_(SCE) for about 200 hours did not appear to lead to a lossof Sn—C particles from the CFP support. While not wishing to be bound byany particular theory, the overall loss of total current density,i_(TOTAL) may be attributable to the loss of Sn—C particles due to lackof adhering which may be corrected with improved binder content.(Compare, Electrode Experiment S3.)

Sn—C-3 Electrode Experiment S3

In Sn—C-3 Electrode Experiment S3, the binder content was increased to 1percent in an effort to reduce Sn—C particle loss from the CFPsubstrate. (Compare, Sn—C-3 Electrode Experiment S2.) In addition,cathode potential, V_(CATHODE) is held at −1.66 V_(SCE) to avoid Sn—Cparticle loss. (Compare Sn—C-3 Electrode Experiment S1, FIGS. 14 and15-18.) The results of Sn—C-3 Electrode Experiment S3 are shown in FIGS.24 and 25-28. Loss in total current, i_(TOTAL) over 240 hours was 10percent and FE remained fairly constant at about 70 percent. Looking atFIGS. 25-28, there seems to be no apparent loss in total Sn—C-3particles. (Compare, Sn—C-3 Electrode Experiment S1, FIGS. 14 and 15-18at higher V_(CATHODE).) Still looking at FIGS. 25-28, there does appearto be some agglomeration and possible breaking up of Sn—C-3 particles.Total Sn—C-3 loss seems negligible, however, thus allowing for the useof such electrodes for longer times at lower applied V_(CATHODE).

While certain embodiments of the present invention have been disclosedin detail, it is to be understood that various modifications may beadopted without departing from the spirit of the invention or scope ofthe following claims.

We claim:
 1. An apparatus for the electrochemical reduction of carbondioxide to formate, comprising: an anolyte compartment, the anolytecompartment at least partially defined by an anode and a membrane; acatholyte compartment, the catholyte compartment at least partiallydefined by the membrane and a cathode, the cathode comprising: a poroussubstrate; and a catalytic coating at least partially covering thesubstrate, the catalytic coating having been affixed to the substrate bydrying thereon an alcohol-based mixture of a catalyst powder comprisingtin-carbon particles, the tin-carbon particles comprising 0.1 nm to 10nm tin deposits on carbon support and a polymeric binder; and a gascompartment, the gas compartment in fluid communication with thecathode.
 2. The apparatus of claim 1, wherein the substrate is carbonfiber paper.
 3. The apparatus of claim 2, wherein the catalyst loadingis between 0.1 mg/cm² and 10 mg/cm², and the binder content is between0.1 to 5 weight percent of the catalyst loading.
 4. A process,comprising: (a) feeding an appropriate anolyte into the anolytecompartment of the apparatus of claim 1; (b) feeding an appropriatecatholyte into the catholyte compartment of the apparatus of claim 1;(c) feeding carbon dioxide into the gas compartment of the apparatus ofclaim 1; and (d) impressing a voltage between the anode and the cathode.5. The process of claim 4, wherein the impressed voltage is sufficientto provide a V_(CATHODE) range between −1.5 and −2.0 V_(SCE).
 6. Theprocess of claim 5, wherein the Faradaic Efficiency of carbon dioxide toformate is about 55 to 95 percent over a period of over 100 hours. 7.The process of claim 4, wherein the cathode comprises carbon fiberpaper.
 8. The process of claim 4, further comprising the step ofrepeating Step (ii) and Step (iii) until a desired coating weight isachieved.
 9. The process of claim 8, wherein the catalyst loading isbetween about 0.1 mg/cm² and about 10 mg/cm².
 10. The process of claim4, wherein the current density is greater than about 100 mA/cm².